The general approach to add touch sensing capability to an organic light emitting diode (OLED) display panel is to supplement a transparent touch sensor layer made from Indium Tin Oxide (ITO) on top of the OLED display panel. To achieve so, there are two possible implementations: 1) Put the transparent touch sensor layer on top of the substrate glass. This is called on-cell touch sensor arrangement; 2) Put the transparent touch sensor layer in-between the cover glass and the substrate glass. This is called in-cell touch sensor arrangement.
The on-cell touch sensor arrangement, as shown in FIG. 1, has the touch sensor layer isolated from the anodes by a thick layer of lower substrate glass (normally, 0.4 mm˜0.7 mm). The capacitive coupling between the anodes and the touch sensors is relatively low. This is good for the touch sensors to sense approaching fingers as this allows a relatively large dynamic range in sensing the changes of capacitance caused by finger touches. The display driver is located on the inner side of the substrate glass. This is called Chip on Glass (COG) arrangement which is a commonly used assemble technique. A touch-sensing controller needs to communicate with the display driver and connected to the touch sensors. Hence, a Chip on Film (COF) arrangement is used to bridge the touch sensors to the touch-sensing controller.
On the other hand, the in-cell touch sensor arrangement as shown in FIG. 2 has the touch sensor layer put in-between the upper cover glass and the lower substrate glass. The touch sensor layer is isolated from the anodes by a very thin layer of insulator (0.5 um˜1.0 um). Hence, the capacitive coupling between the anodes and the touch sensors is high. This parasitic capacitance is much bigger than the induced capacitance coming from an approaching finger, thus causing a poor dynamic range in sensing the changes of capacitance caused by finger touches. However, display driver and touch-sensing controller integration (putting both functions on the same integrated circuit) is feasible since the integrated circuit (IC), touch sensors, and anodes are all located on the inner side of the lower substrate glass.
A more compact approach of in-cell touch sensor arrangement as shown in FIG. 3 is to merge the touch sensor layer with the anode layer so that the anodes are used for both display-driving and touch-sensing. In this arrangement, the display driving and touch sensing functions are time-multiplexed. That is, within a duty cycle, the panel is either in display driving mode or in touch sensing mode but not both. In a typical application for a PMOLED display with a frame refresh rate of around 100 Hz, the display driving mode may take up 90% of the duty cycle while the touch sensing mode may take up 10% of the duty cycle. However, similar to aforementioned in-cell touch arrangement, the anode layer and the cathode layer are in close proximity as the OLED stack layer is only 1 um thick. Hence, the capacitive coupling between the anode layer and cathode layer is high, resulting in a parasitic capacitance which is much bigger than the induced capacitance coming from an approaching finger.
FIG. 4 shows a layout arrangement for electrodes in a general PMOLED display panel (with the display facing the reader). A lower layer consists of an array of cathodes in strip forms running horizontally. An upper layer consists of an array of anodes in strip forms running vertically. A OLED material layer (not shown in the FIG. 4) is held in-between the anode layer and cathode layer while one side of the OLED material connects to the cathodes and the other side of the OLED material connects to the anodes.
The OLED material being held in-between the anode layer and the cathode layer can be regarded electrically as an array of diodes. FIG. 5 shows a circuit model of a PMOLED. A diode has a p-n junction, which is the interface of p-type material and n-type material. A non-forward biased p-n junction can store electric charge at the depletion region. The p-type and n-type materials function like conducting plates of a capacitor while the depletion region acts like the dielectric material of a capacitor. Hence, a real diode can be represented by an ideal diode plus a capacitor in parallel.
In a PMOLED pixel, the electric field at the depletion region is so strong that it is equivalent to an air gap parallel plate capacitor (i.e. air as dielectric) with air gap 0.25 um thick. As mentioned previously, the substrate glass is 0.4 mm˜0.7 mm thick. Another layer of protective glass on a portable electronic device (e.g. smart watch) can be 1 mm˜2 mm thick. Hence, an approaching finger can be 2 mm away from the anode layer (the touch sensing layer) while the cathode layer is effectively 0.25 um away from the anode layer only.
FIG. 6a shows a prior application of configuring and grouping anodes for touch sensing in a PMOLED display panel. The anodes are grouped into 3 groups to act as three touch-sensing keys, Key 1, Key 2, and Key 3 (also indicated as Ch1, Ch2, and Ch3 in the diagram). FIG. 6b shows an electrical model of the PMOLED display panel being touched by a finger. The induced touch sensing capacitance coming from an approaching finger on Key # is represented by CTS#, while the capacitive coupling between the anode layer and cathode layer under Key # is represented by CACC#. As mentioned, the distance between finger and anodes are far (˜2 mm) while the distance between anodes and cathodes are close (˜0.25 um). The ratio between CACC# and CTS# can be in the order of 8000:1 even if the finger is exactly on top of the touch-sensing key. As shown in FIG. 6b, CTS1, CTS2 and CTS3 are connected through CACC1, CACC2 and CACC3 respectively, it is virtually a short circuit. Therefore, the detection of approaching fingers would be obstructed.
With reference to FIG. 7a, the situation is worsen when the display is exposed to sunlight. Sunlight is a very strong light source. When photons with sufficient energy hit the OLED material, electron-hole pairs are created, which is known as photoelectric effect. The electrons move toward the cathode and the holes travel toward the anode; a photocurrent is produced and hampered the touch-sensing signals. In fact, this photoelectric effect is utilized in photovoltaic solar cells in which sunlight is converted to electricity. The impact to the touch sensing in a OLED display panel by this photoelectric effect is further illustrated in FIG. 7b with the additional current sources representing the photocurrent to the electrical model. There are two current paths passing through the touch-sensing controller; one from an approaching finger and the other from the current source representing the photocurrent. The photocurrent varies as the intensity of sunlight exposure varies. As indicated in FIG. 7b, each cathode forms a parasitic capacitor, CCP#, with the back ground of the device comprising the PMOLED touch-sensing display panel. These parasitic capacitors can be small (in the order of 1 to 5 pF) when the back plate of the PMOLED panel is far away (10 mm for example) from the rest of the electronics in the device. Conversely, these parasitic capacitors can be appreciable (in the order of 10 to 30 pF) when the back plate of the PMOLED touch-sensing display panel is close (less than 2 mm for example in a portable device) to the rest of the electronics in the device. If these parasitic capacitors, CCP#, are small then the photocurrent will be small. On the other hand, if these parasitic capacitors, CCP#, are appreciable, the photocurrent may interfere with the sensing of finger touch current. One way to remove this noise current is to block any possible return path of the photocurrent.